278 Mr, G. A. Hemsalech : Excitation of Spectra of Carbon, 

 potential gradient along the tube of about 1*5 ^. Further- 

 more, Dr. King's furnace, unlike mine, is worked with 

 alternating current, and the instruments employed in 

 measuring volts and amperes give, of course, only the root- 

 mean-square values of these quantities. Now, the value for 

 the potential gradient along the tube reaches two maxima 

 during each complete cycle of an alternation, which are 

 equal to 1*4 times the root-mean-square value as read on the 

 voltmeter. In the case of Dr. Kino's furnace this amounts 



volts 



to 1*4 x 1*5 = 2*1 — . for a furnace temperature of between 



2600° and 2700° C. In like manner, at a temperature of 

 between 1800° and 1900° C, a potential of 15 volts has to be 

 applied in order to drive the necessary heating current 

 through the tube, and the maximum value of the potential 



gradient works out to 1*05 ™\ For the sake of con- 

 venience the numbers found for the potential gradients 

 along the furnace tubes in the two cases are restated in 



the following table 



Continuous current 

 tube well protected 



Alternating current 



with carborundum tube 



powder. unprotected. 



Temperature. (Hemsalech.) (Dr. King.) 



1800-1900° C. Not recorded. 1-05— 8 . 



cm. 



2600-2700 „ 1-05 T ~ 2-1 „ 



These figures reveal the fundamental difference between 

 Dr. King's and my furnace — namely, the acting electric 

 field is twice as powerful in the unprotected tube run with 

 alternating current as in the protected tube worked with 

 continuous current. Already at the lower temperature 

 of between 1800° and 1900° C, Dr. King's furnace gives an 

 electric field of such strength as mine attains only at between 

 2600° and 2700° 0. Hence it is quite evident that for a 

 given furnace temperature thermelectronic currents will 

 reach a much higher development in Dr. King's furnace 

 than in mine, and we must, therefore, expect that his 

 furnace will show spectrum lines and bands which cannot 

 be seen in mine under the same temperature conditions. 

 Thus Dr. King's furnace, thanks to its stronger electric 

 field, will show lines at the low temperature which my 

 furnace, owing to its feebler electric field, only shows at the 

 high temperature. This is precisely what has been observed 



